1. Field of the Invention
The United States produces more than 6 billion lb. of plastic film each year, virtually all of which is made from petroleum based raw materials. These films are made most economically by the extrusion blowing process, in which a tubular extruded bubble is expanded and shaped by an air stream at the exit of the extruder die.
In many packaging applications, it is desirable that a film be shrinkable so that it can be made to fit the contours of irregularly shaped articles. Shrink films are now widely used in the packaging industry. Shrinkability arises from an elastic memory imparted to the film during manufacture, and shrinkage takes place when the film tends to revert back to its original unoriented state.
Use of annually renewable, agriculturally derived products such as starch as extenders and replacements for synthetic, petroleum based polymers is currently an active area of research. Use of polysaccharides in plastics not only reduces our dependence on petrochemical-derived monomers, but also reduces problems associated with disposal. The polysaccharide portion will biodegrade, causing the finished plastic article to lose its integrity and be reduced to particles small enough to be of minimal damage to the environment. This invention relates to the preparation of films by the extrusion-blowing of formulations based on a renewable resource, i.e., cereal flour.
2. Description of the Prior Art
In U.S. Pat. No. 4,026,849, Bagley et. al. teach the preparation of plastic articles by extrusion of compositions comprising graft copolymers of starch and thermoplastic polymers such as poly(methyl acrylate) (PMA). The resultant products are water-resistant, yet biodegradable; and they conserve valuable petroleum resources, because the starch portion serves as an extender for the petroleum-based polymer. Dennenberg et al. [J. Appl. Polym. Sci. 22: 459-465 (1978)] teach that starch graft poly(methyl acrylate) copolymers (S-g-PMA) can be extruded into products having excellent tensile strength and elongation properties.
In U.S. Pat. No. 4,839,450 (incorporated herein by reference), Fanta and Otey teach the preparation of extrusion blown films from graft copolymers of starch and poly(methyl acrylate). To obtain continuous films by extrusion blowing, it is essential that the graft polymerization reaction be carried out with a starch or starch derivative that has been rendered completely water soluble prior to the graft polymerization reaction, as opposed to a water suspension of starch granules or water-swollen starch granule fragments. Graft copolymers prepared from water-insoluble starch cannot be extrusion-blown because the presence of starch granules or granule fragments causes the undesirable formation of holes and imperfections in the film during extrusion blowing. Since a film with holes and imperfections cannot be properly expanded by internal air pressure as it exits the die, an inferior film of this type cannot be processed by the extrusion blowing technique. Films prepared according to U.S. Pat. No. 4,839,450 shrink when exposed at room temperature to relative humidities approaching 100%. These films have potential application in shrink-wrapping irregularly-shaped articles without applying heat to induce film shrinkage. Another advantage of these films is that they can be easily removed by water washing.